When manufacturing a bonded SOI wafer by an ion-implantation delamination method, processing such as formation of an insulator film, hydrogen ion implantation, bonding, a delamination heat treatment and others are required. In an SOI wafer after the delamination heat treatment in particular, a defect in a bonding interface called a void or a blister is generated. This defect is strongly dependent on the delamination heat treatment and a previous process thereof. As one of causes, there is a particle that adheres during each process. In particular, defects tend to be generated more often as a thickness of a buried insulator film (BOX) is reduced.
When fabricating an SOI wafer, there is a tendency that the number of defects increases as a thickness of a BOX decreases to 100 nm or below, and defects are generated due to, e.g., particles in a previous process including a delamination heat treatment even if the BOX is as thick as 100 nm or above.
As such defects, there are a blister and a void that can be visually confirmed, and an LPD (Light Point Defect) that is detected by a particle counter, and others. However, using SEM and others for observation, an actual figure of the LPD is a small void. These defects must be reduced and eliminated as much as possible, and, in particular, defects in an SOI wafer having a thin BOX or in a direct bonded wafer having no BOX where defects are apt to be generated must be reduced.
To reduce defects, although there is a method of improving rigidity by increasing an implantation depth of a hydrogen ion to enlarge a thickness of an SOI layer, an effect thereof becomes insufficient when a thickness of the BOX is small. Further, when the hydrogen ion is deeply implanted, an amount of reducing a thickness of the SOI layer by, e.g., sacrificial oxidation in a post-process increases, a process time is prolonged, and an SOI film thickness distribution tends to be degraded.
As another method of reducing defects, there is also a method of performing a plasma treatment for exposing a bonding surface to a plasma to activate the bonding surface, thereby improving bonding strength. As a standard process thereof, plasma activation+aqueous cleaning+bonding is recommended. For example, Patent Document 1 discloses a method of performing a plasma treatment to form an oxide film, cleaning a surface of this film with pure water, drying the same and then effecting bonding (see Japanese Patent Application Laid-open No. 05-82404).
However, a reduction in defects (voids or blisters) in a bonding interface is not sufficient even though a bonded wafer is manufactured based on such a method.